The present invention relates to certain organometallic polymers which are useful as photoresists and which exhibit enhanced resistance to plasma and especially to Cl2/O2 plasma used in reactive ion etching. The present invention is concerned with the compositions-as well as their use in lithography. For instance, the materials of the present invention are suitable for use in device and mask fabrication on optical, e-beam, x-ray and ion-beam lithography tools.
In the manufacture of patterned devices and especially microelectronic devices, the processes of etching different layers which constitute the finished product are among the most crucial steps involved. One method widely employed in the etching process is to overlay the surface to be etched with a suitable mask.
The mask is typically created by imagewise forming a pattern of resist material over those areas of the substrate to be shielded from the etching. The resist is normally formed of a polymeric organic material. The pattern is formed by imagewise exposing the resist material to irradiation by lithographic techniques. The irradiation employed is usually x-ray, UV radiation, electron beam radiation or ion-beam radiation.
Radiation sensitive materials and/or compositions are either positive-acting (i.e. radiation solubilizable) or negative-acting (i.e. radiation insolubilizable or radiation crosslinkable). Positive-working (radiation) sensitive compositions are rendered soluble (or developable) by actinic radiation (deep-near UV, x-ray, electron-beam or ion-beam) and can be removed using selective developing solutions leaving unexposed areas intact. Negative-working (radiation) sensitive compositions are those which become insoluble upon exposure to actinic radiation. Selected solutions can dissolve and remove the unexposed areas of the composition while leaving the exposed portions intact. Development of such exposed materials yields negative tone images.
After the resist is developed forming the desired mask, the substrate and mask can be immersed in a chemical solution which attacks the substrate to be etched while leaving the mask intact. These wet chemical processes suffer from the difficulty of achieving well-defined edges on the etched surfaces. This is due to the chemicals undercutting the mask and the formation of an isotropic image. In other words, conventional chemical wet processes do not provide the resolution considered necessary to achieve optimum dimensions consistent with current processing requirements.
Moreover, such wet etching processes are undesirable because of the environmental and safety concerns associated therewith.
Accordingly, various so-called xe2x80x9cdry processesxe2x80x9d have been suggested to improve the process from an environmental viewpoint, as well as to reduce the relative cost of the etching. Furthermore, these xe2x80x9cdry processesxe2x80x9d have the potential advantage of greater process control and higher aspect ratio images. Also, when fabricating patterns having feature sizes below 350 nm, dry etching processes are necessary for profile control.
Such xe2x80x9cdry processesxe2x80x9d generally involve passing a gas through a container and creating a plasma in this gas. The species in this gas are then used to etch a substrate placed in the chamber or container. Typical examples of such xe2x80x9cdry processesxe2x80x9d are plasma etching, sputter etching, and reactive ion etching.
Reactive ion etching provides well-defined, vertically etched sidewalls.
One of the challenges in the fabrication of microelectronic devices and masks is to develop a resist which exhibits good lithographic performance as well as high dry etch resistance for subsequent pattern transfer into an underlying substrate. The dry etch chemistries include O2 currently used for antireflective coatings, Cl2/O2 currently used for chrome etching in mask fabrication, Cl2 based plasma for polysilicon etch, and fluorocarbon based plasmas such as CF4 for oxide etching. These plasmas are examples only and are not meant to limit the scope. Conventional novolak/diazonapthoquinone resists used for i-line lithography have to date exhibited the best dry etch resistance. ZEP is an e-beam resist which has been adopted by the industry for advanced mask making to replace the conventional wet etch polybutenesulfone (PBS) process. Although ZEP provides significant improvement over the PBS process, its dry etch resistance to Cl2/O2 is marginal (etch rate of 1.95 nm/s). Novolac is 1.4 nm/s.
There is a need to develop radiation sensitive compositions that provide improved dry etch resistance for use in mask fabrication (binary, attenuating phase shift masks, alternating phase shift masks) and for device fabrication.
Further, the use of polyvinyl diphenylferrocene and polyvinylferrocene as negative resists in ion implantation masking and in the formation of conductive patterns has been suggested (see U.S. Pat. No. 3,885,076). It is stated that the electron beam causes crosslinking of these polymers and renders negative patterns. Other polymers containing metal groups are referred to in U.S. Pat. No. 4,156,745. Lead methacrylate, when incorporated into copolymers with methylmethacrylate increases the speed of the resist compared to homopolymers of methylmethacrylate. The use of polyvinylferrocene has been proposed for oxidative decomposition to iron oxide patterns according to U.S. Pat. No. 4,027,052. The pattern delineation is accomplished by applying x-rays. Moreover, silicon containing resists have been quite prevalent. The use of silicon and germanium has been intended to impart O2 etch resistance to the resist material. For example, see U.S. Pat. Nos. 4,764,247; 4,935,094 and 5,733,706, and Microelectronic Engineering 3,279 (1985). Nevertheless, the prior art does not disclose any organometallic polymeric materials for masking against C2 and Cl2/O2 RIE (reactive ion etching).
Cl2 RIE and Cl2/O2 RIE are used in the electronics industry in etching polysilicon and in etching chromium in optical mask fabrication. The etched patterns are differentiated from the unetched surface areas by a layer of resist material. The resist should have adequate resistance towards the particular plasma used in the etching. Cl2/O2 plasma is regarded as one of the harshest environments to which a surface can be subjected. Examples of resists that are used for the above-mentioned applications are e-beam resists such as ZEP 7000 and ZEP 520 used for patterning chromium in the preparation of optical masks and copolymers of tert.-butyl methylacrylate, tert.-butyl methacrylic acid, and methylmethacrylate, experimental resists, referred to herein as xe2x80x9cXxe2x88x921xe2x80x9d, described in U.S. Pat. No. 5,071,730 and intended as a 193 nm UV resist. Other deep UV resists that are used for patterning in Cl2/O2 plasma include UV2 and UV6. ZEP resist has the chemical formula 1 shown below. 
The etch rate of ZEP in Cl2/O2 plasma is 1.95 nm/sec compared to 1.40 nm/sec for Novolac. This rate is regarded as marginal. The structure of Xxe2x88x921 is shown below as formula 2. It also has a low resistance to Cl2/O2 plasma because of the methacrylate backbone of the polymer. 
Both ZEP and Xxe2x88x921 resists require an increase in their RIE resistance properties. Other commercial resists such as amplified resists (e.g. APEX E) for deep UV applications, could also benefit from a boost in their RIE resistance properties.
The present invention provides compositions which are especially useful as resists and which are sensitive to imaging irradiation while exhibiting enhanced resistance to reactive ion etching. The compositions of the present invention typically exhibit enhanced resistance to reactive ion etching using in particular Cl2/O2 plasma, Cl2 plasmas, O2 plasmas and fluorocarbon plasmas.
In particular, the polymers of the present invention are from at least one monomer selected from the group consisting of an organometallic acrylate, organometallic methacrylate, organometallic styrenes and mixtures thereof wherein the metal is selected from the group consisting of yttrium, aluminum, iron, titanium, zirconium, hafnium and mixtures thereof.
The present invention also relates to a method for forming a pattern of a resist which comprises:
a) providing on a substrate a layer of a resist composition which comprises a resist from at least one monomer selected from the group consisting of an organometallic acrylate, organometallic methacrylate, organometallic styrenes and mixtures thereof wherein the metal is selected from the group consisting of yttrium, aluminum, iron, titanium, zirconium, hafnium and mixtures thereof.
b) imagewise exposing the layer of resist composition to irradiation; and
c) developing the resist to thereby form the pattern.
A further aspect of the present invention relates to forming a pattern on a substrate which comprises:
a) providing a layer to be patterned on a substrate,
b) providing on the layer to be patterned a layer of a resist composition which comprises a photoresist from at least one monomer selected from the group consisting of an organometallic acrylate, organometallic methacrylate, organometallic styrenes and mixtures thereof wherein the metal is selected from the group consisting of yttrium, aluminum, iron, titanium, zirconium, hafnium and mixtures thereof.
c) imagewise exposing the layer of resist composition to irradiation,
d) developing the resist to form the desired pattern, and
e) subjecting the layer to be patterned to reactive ion etching with the resist acting as a mask to thereby form the desired pattern on the substrate.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described only the preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.
The polymers employed according to the present invention are obtained from polymerization of at least one organometallic monomer selected from the group consisting of an organometallic acrylate, organometallic methacrylate, organometallic styrenes and mixtures thereof wherein the metal is selected from the group consisting of yttrium, aluminum, iron, titanium, zirconium, hafnium and mixtures thereof. The styrenes include substituted styrenes such as acetoxystyrene. Preferably at least a two-carbon atom chain exists between the metallic moiety and acrylate or methacrylate moiety.
The preferred metallic moiety is iron. Both homopolymers and copolymers are contemplated according to the present invention.
The polymers typically have average number molecular weights of about 2000 to about 100,000, and preferably about 4500 to about 10,000.
Examples of such suitable organometallic monomers are 2-ferrocenylethyl acrylate, 2-ferrocenylethyl methacrylate, 2-(ferrocenylmethyl-2-urethano)-ethyl methacrylate, 2(ferrocenylmethyl-2-urethano)-ethyl acrylate, 4-ferrocenyl methylstyrene, and 3 ferrocenylmethyl 4-acetoxy styrene.
In the case of copolymers, the other monomer is typically at least one monomer selected from the group consisting of acrylic acid, methacrylic acid, esters thereof and mixtures thereof.
Examples of suitable monomeric esters are 2-hydroxyalkyl methacrylates and 2-hydroxyalkyl acrylates wherein the alkyl has 1-10 carbon atoms; alkylacrylates and alkylmethacrylates thereof wherein the alkyl has 1-10 carbon atoms; and chloroalkyl acrylates and chloroalkyl methacrylates wherein the alkyl has 1-10 carbon atoms.
The copolymers typically contain about 1% by weight to about 90% by weight, more typically about 10% by weight to about 70% by weight, and preferably about 20% by weight to about 50% by weight, of the organometallic monomer and correspondingly about 99%. to about 10% by weight, anbout 90% to about 30% by weight and about 80% by weight to about 50% by weight of the other comonomer(s).
In addition, copolymers can further include other monomers such as styrene, alphamethyl styrene, 4-hydroxy styrene, acetoxy styrene and other derivatives of hydroxystyrene.
Examples of some acrylates and methacrylates are:
2-hydroxyethyl methacrylate
2-hydroxypropyl methacrylate
2-hydroxyethyl acrylate
2-hydroxypropyl acrylate
2-hydroxybutyl acrylate
2-hydroxypentyl acrylate
methylmethacrylate
4-butylmethacrylate
methylacrylate
alphachloromethylacrylate
t-butyl methacrylate
t-butyl acrylate.
Examples of some copolymers according to the present invention are:
copolymers of alphachloromethyl acrylate and 2-ferrocenylethyl acrylate,
copolymers of alphachloromethyl acrylate and 2-ferrocenylethyl methacrylate,
copolymers of alphachloromethylacrylate, alphamethyl styrene and 2-ferrocenylethyl acrylate,
copolymers of alphachloromethyl acrylate, alphamethyl styrene and 2-ferrocenylethyl methacrylate,
copolymers of 2-ferrocenylethyl acrylate, methacrylic acid and t-butyl methacrylate,
copolymers of 2-ferrocenylethyl methacrylate, methacrylic acid and t-butylmethacrylate,
copolymers of alphachloromethyl acrylate and 2-(ferrocenylmethyl-2-urethano-)-ethyl acrylate,
copolymers of alphachloromethyl acrylate and 2-(ferrocenylmethyl-2-urethano-)-ethylmethacrylate,
copolymers of alphachloromethylacrylate, alphamethyl styrene and 2-(ferrocenyl-methyl-2-urethano-)-ethyl acrylate,
copolymers of alphachloromethylacrylate, alphamethyl styrene and 2-(ferrocenyl-methyl-2-urethano-)-ethyl methacrylate,
copolymers of 2-(ferrocenyl methyl-2-urethano-)-ethyl acrylate, methacrylic acid and t-butyl methacrylate,
copolymers of 2-(ferrocenylmethyl-2-urethano-)-ethyl methacrylate, methacrylic acid and t-butyl methacrylate,
copolymers of 2-ferrocenylethyl acrylate, xcex1-methyl 4acetoxystyrene and t-butyl methacrylate,
copolymers of 2-ferrocenylethyl methacrylate, xcex1-methyl 4-acetoxystyrene and t-butylmethacrylate,
copolymers of 2-(ferrocenyl methyl-2-urethano-)-ethyl acrylate, xcex1-methyl 4-acetoxystyrene and t-butyl methacrylate,
and copolymers of 2-(ferrocenylmethyl-2-urethano-)-ethyl methacrylate, xcex1-methyl-4-acetoxystyrene and t-butyl methacrylate.
In the case of the above terpolymers, the amount of the organometallic monomer is typically about 1 to 30% by weight and more typically about 1 to about 20% by weight with the other comonomers correspondingly about 70 to about 99% by weight and about 80% to about 99% by weight.
For example, terpolymers of alphachloromethyl acrylate, alphamethyl styrene and the organometallic monomer typically contain about 1-50% of the acrylate and about 1-80% of the organometallic monomer. Terpolymers of the organometallic monomer, methacrylic acid and t-butyl methacrylate typically contain about 1 to about 25% of the organometallic monomer, about 5 to about 30% of the methacrylic and about 45 to about 90% of the t-butyl methacrylate.
An example of modification of Xxe2x88x921 by incorporating ferrocenyl groups is illustrated by formula 3 below. 
In addition, the polymers of the present invention can be admixed with other resists to increase the RIE resistance of the other resists such as ZEP and Xxe2x88x921. For instance, polymers of (1-ferrocenyl-2-oxyethane) methacrylate, including homopolymers thereof, can be blended with other resists. When used in mixture, the amount of the polymers of the present invention are typically about 90 to about 1% by weight, more typically about 70% by weight to about 10% by weight, and preferably about 50% by weight to about 20% by weight and, correspondingly, the other resist is about 10 to about 99% by weight, about 30 to about 90% by weight and about 50% by weight to about 80% by weight.
The polymers of the present invention are typically prepared by free radical solution polymerization using a free radical catalyst such as AIBN and solvent such as tetrahydrofuran, isopropanol and toluene.
The polymerization is usually carried out at reflux temperature of the solvent.
The polymers of the present invention can be developed employing an organic solvent such as ethyl-3-ethoxypropionate.
The following illustrates a typical fabrication sequence for providing optical masks for microlithography by providing a pattern of chromium metal on a quartz plate by the following steps:
1. A thin film of chromium metal is provided on the surface of a quartz plate.
2. The metal layer is coated with e-beam resist.
3. The resist is patterned by e-beam.
4. The plate is developed in a suitable developer.
5. The exposed chromium film is etched either by wet etch or by dry etch.
6. The residual resist is removed.
The following non-limiting examples are presented to further illustrate the present invention.